1. Field of the Invention
The present invention relates to a method of depositing a silicon dioxide film and, more particularly, it relates to an improved method of depositing a silicon dioxide film on the surface of a substrate by bringing the substrate into contact with an aqueous solution of hydrosilicofluoric acid supersaturated with silicon dioxide (hereinafter referred to as "liquid phase deposition method").
2. Description of the Prior Art
A conventional method of depositing a silicon dioxide film on the surface of any substrate is described in Japanese Patent Application Laid-open No. 1982/196744. According to this method, hydrosilicofluoric acid having a concentration of 0.5 to 3.0 mole/l is saturated with silicon dioxide, and then, boric acid is added to the hydrosilicofluric acid solution saturated with silicon dioxide in the amount of 2.0.times.10.sup.-2 mol or more per 1 of the hydrosilicofluoric acid solution to prepare a treating solution which is supersaturated with silicon dioxide, and a substrate is dipped in the treating solution. Another conventional method of depositing a silicon dioxide film on the surface of any substrate is described in Japanese Patent Application Laid-open No. 1983/161944. According to this method, the supersaturation state with silicon dioxide is maintained for a certain period by continuously adding boric acid to the above-mentioned treating solution (Patent application laid-open No. 1983/161944).
The above conventional liquid phase deposition methods have advantages that the deposition of the film is possible at a low temperature, and that it is possible to deposit on substrates of any material and any shape, but it has a disadvantage, on the other hand, that a complicated process is required for the detoxifying treatment of the effluent of the used treating solution to be discarded.
In a fluorine-containing effluent, fluorine is generally separated and removed as CaF.sub.2 precipitate formed by adding Ca(OH).sub.2.
The concentration of fluorine in the effluent which is produced in the above-mentioned liquid phase deposition methods cannot readily be decreased, however, by single addition of Ca(OH).sub.2, and it is therefore necessary to repeat many times the cycle of addition of Ca(OH).sub.2, precipitation, and separation.